In ITS (Intelligent Transport Systems), radio waves are used in performing intervehicle communication to exchange information between vehicles. In intervehicle communication that is carried out by using radio waves of a predetermined band, an increase in the number of vehicles that are carrying out intervehicle communication and that are present within a certain area leads to congestion of radio waves, raising the concern that communication between vehicles will become impossible.
In communication between portable telephones carried out by way of base stations, base stations typically implement control to avoid congestion. However, intervehicle communication lacks base stations carrying out control to avoid congestion. As a result, when the number of vehicles present within a certain area that are carrying out intervehicle communication increases, control must be implemented to avoid congestion between the vehicles.
WO2008/099716 (Patent Literature 1) discloses a vehicular communication device that implements control for avoiding congestion.
The vehicular communication device described in Patent Literature 1 estimates the congestion state level that indicates whether the occurrence of congestion is impending or not, based on the channel utilization factor. The channel utilization factor represents the proportion of transmitting and receiving channels that are being used.
The vehicular communication device described in Patent Literature 1, when the congestion state level is equal to or greater than a predetermined level, decreases the transmission power of its own vehicle to a value which is less than the transmission power of its own vehicle at a time of the congestion state level below the predetermined level.